Relaxation dynamics in glass forming liquids with related molecular structures Zeming Chen a , Dongyang Bi a , Riping Liu a , Yongjun Tian a , Li-Min Wang a,⇑ , Kia L. Ngai b a State Key Lab of Metastable Materials Science and Technology, and College of Materials Science and Engineering, Yanshan University, Qinhuangdao, Hebei 066004, China b Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy article info Article history: Received 9 July 2012 In final form 12 September 2012 Available online 19 September 2012 abstract The relaxation dynamics of a series of molecular liquids with modified structures from aldehyde with a fixed number of carbon atoms is studied. Structural modification is made by introducing oxygen into the main chain, or by incorporating end group moieties such as ethyl, acrylate, methacrylate and dihydroxyl. Broadband dielectric measurements were performed on the glass-formers. The experimental results emphasize the importance of intermolecular interactions and molecular rigidity in determining the kinetic fragility and non-exponential parameters of the structural a-relaxation. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Dynamic and thermodynamic properties of glass-forming liq- uids are key experimental facts for understanding the mechanism of glass transition. Benefitted by the development of a variety of experimental techniques with broad ranges of time and frequency, data on the dynamics of many glass-formers can be acquired over many decades of time or frequency covering primary and second- ary relaxations [1–14]. The dynamic properties depend on the structures of the glass-formers, and it would be illuminating to find out whether chemical structure bears any connection to dynamics properties. With the correlations, one can go one step further to gain insight into the glass transition problem, which has attracted considerable interest in the exploration of theories and models [9,15–20]. In order to achieve systematic changes of chemical and physical structures, glass-formers within the same family that are closely related must be chosen to compare their dynamic prop- erties. Examples of such experimental studies in the past decades include (1) the carbon backbone polymers with different chemical structure of the repeat units [21], (2) the poly(n-alkyl methacry- lates) with alkyl side chain of increasing length [22], (3) the polyols on increasing the number of carbon atoms [23], (4) the nano-phase separated domains of different sizes formed by side chains of poly(n-alkyl methacrylates) with different lengths [24,25], (5) the oligo(propylene glycol) dimethyl ethers with different numbers of repeat units of propylene glycol [26]. (6) isomers [27–29], and (7) molecular homologous series with the various alkane branches such as ethyl-, propyl- and butyl benzene [30] and di-methyl-, di- ethyl- and di-butyl-phthalate [31]. In this Letter, we have chosen some small molecular glass-form- ers with chemical and physical structures that can be related to each other. The changes in dynamic properties with systematic changes in structure are examined with the purpose of finding cor- relation between the two, and identifying the attribute of the chemical structure from which the correlation with the dynamic properties originates. Specifically, we start from aldehyde with a fixed number of carbon atoms and consider various modifications of the chemical structure made by introducing oxygen into the main chain, or by incorporating end group moieties such as ethyl, acrylate, and methacrylate and dihydroxyl. With the number of carbon atoms fixed in all the modification, the effect of molecular rigidity (conformational restriction) and intermolecular interaction potential on the dynamics can be examined. The dynamics of the structural a-relaxation and the secondary -relaxation of all these glass-formers have been deduced from the broadband dielectric relaxation measurements over wide ranges of temperature T and frequency f. The results emphasize the importance of the intermo- lecular interaction force and molecular rigidity in governing the relaxation dynamics. 2. Experimental The glass-formers studied in this Letter are given as follows to- gether with the purity level and the suppliers. (a) 2-ethyl hexanal (>95%, TCI), (b) butyl methacrylate (99%, Alfa Aesar), (c), cyclohexyl acetate (>99%, TCI) and (d) 2-ethyl-1,3-hexanediol, (99%, Acros Organics). Broadband dielectric relaxation measurements were made in a Novocontrol broadband dielectric spectrometer (Con- cept 80) equipped with a liquid nitrogen cooling system. The dielectric relaxation were isothermally measured at each tempera- ture by holding liquid between two separated brass electrodes by Teflon strips of thickness 25 m. The scanning frequency range 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.09.026 ⇑ Corresponding author. E-mail address: Limin_Wang@ysu.edu.cn (L.-M. Wang). Chemical Physics Letters 551 (2012) 81–85 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett